1. Field of the Invention
This invention relates generally to AC-to-DC converter systems. More particularly, this invention relates to an improved controller circuit design and configuration for use with synchronous rectification to achieve high power conversion efficiency.
2. Description of the Prior Art
Conventional art of design and manufacture of AC to DC converter systems is not able to satisfy the more advancing requirements now imposed by the high performance server technology. These requirements include smaller size, lower fan noise, high reliability, low cost and low power consumption. The power supply systems are now employed in the computer industries to convert various AC voltages ranging from one hundred to two hundred and forty volts to regulated DC voltages of 3.3, 5, 12 and xe2x88x9212 volts. Specifically, the difficulties arise from the facts that these requirements appear to constrain the designs of the power supply systems in opposite directions. On the one-hand the power supply is expected to produce more output power and be more reliable. On the other hand, the power supply is constrained by seemingly contradictory requirements that the system be made smaller, quieter, and cheaper. One of the best ways to satisfy these requirements is to increase the efficiency of the power supply system. This is because efficiency improvement would lead to reduction of heat generation thus allow for smaller size of a power supply to operate at a lower temperature that would increase the reliability and meanwhile require less noise generated by fans for heat-dissipation. Although, there may be a concern that a system designed for higher efficiency tends to be more complex, and this increases the production cost, such concerns are likely offset by the follow-on savings in heat dissipation, package, shipping, and cost reductions resulted from lower power consumption.
There are a number of ways to increase the efficiency of a power supply. A method is to reduce the losses in the output rectifier of a converter since these losses are relatively large compared to other losses. In a high frequency power converter as that shown in FIG. 1A, the standard devices for rectifying an output voltage of three to five volts are schottky diodes D1 and D2. FIG. 1A is a generic representation of a basic forward converter showing a conventional circuit configuration of forward switching converter for a power supply system operated with a pulse width modulator controlling a main switching transistor Q1 at the primary side. The Pulse Width Modulator is any one of many commercial integrated circuits, which can modulate a pulse width duty cycle based upon a feedback signal. Its output is an approximately 0 to 12V pulse waveform at a fixed frequency, e.g., a frequency of 100 KHz. This waveform drives the main switching transistor Q1. Transistor Q1 acts as a power switch under control of the Pulse Width Modulator output. With a rectified input voltage source 400 volts DC, the waveform appearing at the output (drain) of Q1 has a peak value of about 400 Volts or a peak-to-peak value of 800 Volts. The primary side is coupled to the secondary side with a transformer T1 with the secondary side provided with rectifying diodes D1 functioning as a forward output diode and D2 as a freewheel output diode. The output load is coupled in series with an output filter inductor and in parallel to an output filter capacitor. For a typical 5 Volt DC output, the transformer has a turns ratio defined by Np:Ns of about 15:1 where Np is the number of turns of the primary side and Ns is the number of turns of the secondary side. The peak voltage into the anode of D1 is around 25 Volts. When Q1 is on, D2 is reverse biased and D1 is forward biased (the anode is positive relative to the cathode). During this time, a positively sloped output current flows through D1 and L1 to the output load. L1 (output filter inductor) stores most of the transformer output energy pulse to produce a ramping current which is usually continuous. When Q1 is off, D1 is reverse biased and L1 maintains a negatively-ramping current by forward-biasing D2 as it discharges some of its stored energy. Except for resistive losses, the average voltage across C1 is identical to the average voltage across D2. C1 serves to filter the periodic and random perturbations from the DC output voltage so as to reduce them to acceptable levels.
As that shown in FIG. 1A, schottky diodes often cause a forward voltage drop of about 0.6 volts. Even if the power supply has no other losses, the voltage drops caused by the shottky diodes represent about ten to fifteen percent efficiency loss for a three to five volt output. In order to compensate for these losses, higher power is required prior to a rectification action taken by the schottky diodes. A higher power processed by prior stages of the power supply system tends to increase losses further during during these prior-stage-processing functions. As a result, the losses are compounded and the total efficiency losses are significantly increased. Conversely, the total power savings achieved by improving the output rectifier efficiency tend to have a reverse effect of compounding the improvement of the efficiency for the entire power supply system. There are methods to incrementally minimize the losses of the schottky diodes by optimizing the transformer and choosing the best schottky diodes. However, improvements of power losses achievable by using better schottky diodes are quite limited. Under the circumstances when a small performance improvement is required, better schottky diodes would generally be sufficient to satisfy the requirement. But when compared to another technique of applying a synchronous rectification, even the best schottky diodes would come short of matching the performance when synchronous rectification is employed for AC to DC conversion.
An effective method to increase the output rectifier efficiency is by implementing a controlled switch to achieve synchronous rectification. In the recent past, synchronous rectification was considered too exotic for commercial applications. The device most commonly used for the controlled switch is a MOSFET. Advancement in semiconductor technology has improved the cost/performance of the MOSFETs, and the power supply industry now begun to use synchronized rectification for performance improvement as the improvements achievable by the schottky are not sufficient to meet the demand of higher performance. The most commonly available switch is an n-channel MOSFET transistor that has an operation characteristic of providing a blocking voltage when the drain is positive relative to the source and the gate is at a zero or negative potential relative to the source. Due to an inherent drain-to body diode, there will be always a current even under a negative gate biased condition when the drain is negative relative to the source. This is normally considered as an undesirable feature for typical applications of the n-channel MOSFET. However, by providing a positive voltage to the gate, e.g., 10 volts relative to the source to turn on the n-channel MOSFET, the n-channel MOSFET will conduct a current with a very low voltage drop. This occurs regardless of the polarity of the voltage applied to the drain relative to the source. The MOSFET transistor thus provides an operation characteristic that is useful to function as a very efficient rectifier. Specifically, the rectifying function is achieved by adjusting the gate-source voltage to negative or zero to prevent a reverse current. And, conversely to generate a low voltage-drop conducting condition by adjusting the gate-source voltage to positive to provide a rectified current. The method is however depends on proper synchronization of the gate voltage to the variations of the relative source-drain potential. FIG. 1B shows a conventional synchronous rectified converter where the synchronization control signal is generated from the primary side. Specifically, a leading dead time (referring to FIG. 3 below) is generated from the control signals for Q1 and applied simultaneously to the synchronous rectifier switching transistor Q2 through a driver circuit. The pulse width modulation output is transmitted across the safety isolation barrier to driver circuit using a transformer or other device with a delay inserted between the PWM signal and main switch Q1 on the primary side. Such control scheme for achieving synchronous rectification has the disadvantages that it is necessary to provide an added signal path across the isolation barrier and usually that requires a bulky and expensive transformer. Also, the circuit of the primary circuit must be modified in order to accommodate this control scheme and that adds to the production cost of the power supply systems. Other than these considerations, the control method using synchronous rectification is employed in high efficiency non-isolated DC-DC converters commonly used to power the central processor unit (CPU) of a computer. However, due to above difficulties and considerations, and the facts that conventional power supply systems can usually tolerate lower efficiency, synchronous rectification is usually not employed in off-line power supply system for AC-DC conversions.
For the above reasons, a need still exists in the art of designing and manufacturing a power supply system with voltage rectifying converter to provide an improved apparatus and method to increase the efficiency of the rectifying operations. Specifically, an improved method to improve the synchronous rectification of a power supply system to achieve lower voltage drop with precise time control of gate voltage synchronization is required to overcome the difficulties and limitations faced by those of skill in the art of the power supply industry.
It is therefore an object of the present invention to provide a novel and improved AC/DC converter to achieve a lower voltage drop for increasing the rectification efficiency with better cross regulation such that the more advanced requirements imposed on a power supply for high-performance servers can be satisfied.
Specifically, a synchronized rectification controller is employed to control a synchronized rectification (SR) switch implemented on the secondary side of the AC/DC converter. The SR switch is implemented as a MOSFET with synchronization rectifier controller control the sequence and timing of the gate voltage of the SR MOSFET in response to the switching on and off of the transformer and the voltage variations of the rectifier diodes used in the AC/DC converter. By precisely controlling the gate voltage of the SR MOSFET to assure an operation of synchronized rectification, higher conversion efficiency is achieved with lower voltage drops and power losses resulted from the AC/DC conversion process.
Another object of this invention is to provide improved synchronous rectification circuit configuration with new control circuit. The new control circuit allows the addition of synchronous rectification to the non-post regulated output of the power supply without any modifications to the transformer or the control of the primary side of the power supply and without any additional bridging to cross over the primary-secondary barrier.
Another object of this invention is to provide improved synchronous rectification circuit configuration with new control circuit to perform the necessary functions with inexpensive, commonly available parts as required circuit elements. The performance improvements are therefore achieved without unduly increasing the production cost of the power supply system.
Briefly, this invention discloses an AC-to-DC converter that includes a transformer having a primary side for inputting an input signal and a secondary side for outputting an output signal. The AC-to-DC converter further includes a synchronous rectifier controller connected to the secondary side for controlling a synchronous rectifier (SR) switch on the secondary side for generating the output signal. The SR switch is implemented as a MOSFET transistor with a gate connected to the synchronous rectifier controller. The synchronous rectifier controller further includes a plurality of circuit elements for turning off the SR switch before a main switch of the transformer is turned on. The synchronous rectifier controller further turns on the SR switch when the main switch of the transformer is turned off. The synchronous rectifier controller controls the SR switch and turns it off with a precisely controlled dead time before the main switch of the transformer is turned on.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various drawing figures.